Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIG. 1, an SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of a conductive material such as titanium for example. The case 12 typically holds the circuitry and battery necessary for the IPG to function, as described in detail below. The IPG 10 is coupled to distal electrodes 16 designed to contact a patient's tissue. The distal electrodes 16 are coupled to the IPG 10 via one or more electrode leads (two such leads 18 and 20 are shown), such that the electrodes 16 form an electrode array 22. The electrodes 16 are carried on a flexible body 24, which also houses the individual signal wires 26 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 18, labeled E1-E8, and eight electrodes on lead 20, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The leads 18, 20 contain proximal electrode contacts 29, which couple to the IPG 10 using lead connectors 28 fixed in a non-conductive header material 30 such as an epoxy.
As shown in the cross-sections of FIGS. 2A and 2B, an IPG 10 typically includes a printed circuit board (PCB) 32 to which various electronic components 34 are mounted, some of which are discussed below. A telemetry (antenna) coil 36 is used to transmit/receive data to/from an external controller 50, as explained further below. In these examples, the telemetry coil 36 is within the case 12, although it can also be placed in the header 30 in other examples. U.S. Pat. No. 8,577,474 discloses telemetry antennas in both of these locations, and is incorporated herein by reference.
IPGs can differ in the type of battery employed. FIG. 2A shows an IPG 10r that contains a rechargeable battery 14r (where “r” denotes “rechargeable”). To facilitate charging of battery 14r, the IPG 10r contains an additional charging coil 37, which wirelessly receives a magnetic charging field 80 from a coil 76 in a hand-holdable and portable external charger 70 (FIG. 2C). Such means of charging battery 14r using an external charger 70 occurs transcutaneously through the patient's tissue 100 via magnetic induction. When the external charger 70 is turned on (switch 84), and referring to FIG. 3A, charging circuitry 94 generates an AC current (Icharge) in coil 76. This produces an AC magnetic charging field 80 (e.g., of 80 kHz), which induces an AC current in charging coil 37 in the IPG 10r. This current is rectified 44 to a DC level used to recharge the battery 14r, perhaps via battery charging and protection circuitry 46. Rechargeable batteries 14r can be formed using different chemistries, but lithium ion polymer batteries are popular for use in implantable medical devices, and can be charged to a battery voltage (Vbr) of about Vbr_max=4.2 Volts (see FIG. 4A) in one example.
IPGs with rechargeable batteries 14r can transmit data to their associated external chargers 70 using Load Shift Keying (LSK), which involves using serial bits to be telemetered (from LSK modulator 40) to modulate the impedance of charging coil 37 (via transistor 42). This manifests as a change in the voltage used by the external charger 70 to produce the AC current (Icharge) in coil 76, and so such voltage can be demodulated 96 and the data bits recovered for interpretation for the external charger 70's control circuitry 92. LSK telemetry is well known, and further details concerning LSK telemetry are disclosed in U.S. Patent Application Ser. No. 61/877,877, filed Sep. 13, 2013, which is incorporated herein by reference.
FIG. 2B shows an IPG 10p that contains a non-rechargeable primary battery 14p (where “p” denotes “primary”). Unlike a rechargeable battery 14r, the electrochemical reaction in a primary battery 14p is not reversible by passing a charging current therethrough. Instead, a primary battery 14p will eventually expend the materials in one or both of its electrodes, and thus has a limited life span. Once the battery 14p is exhausted, it will be necessary to explant IPG 10p from the patient so that the battery 14p can be replaced and the IPG 10p re-implanted, or (more likely) so that a new IPG 10p with a fresh battery 14p can be implanted. Primary batteries 14b can be formed using different chemistries, but Lithium CFx batteries, or Lithium/CFx-SVO (Silver Vanadium Oxide) hybrid batteries are popular for use in implantable medical devices, and produce battery voltages of Vbp_max=1.2-3.2 Volts (see FIG. 4B) for example. Because battery 14p is not rechargeable, there is no need for a charging coil (compare 37 in FIG. 2A) in IPG 10p, and no need for an external charger 70. Structures relevant to charging that would not be used with a primary battery IPG 10p are shown in dotted lines in FIG. 3A.
Regardless whether a rechargeable or primary battery 14r or 14p is used in the IPG 10, that battery ultimately provides the power (Vbr, Vbp) for the bulk of the operative circuits 47 in the IPG 10 via power supply node Vdd, such as analog or digital circuits and their associated regulators. Analog circuits 47 can comprise thermistors, band gap voltage references, oscillators and clocks, modulation 41 and demodulation 43 circuitry (FIG. 3A), analog measurement and routing circuitry, etc. Digital circuits 47 can include the control circuitry 38 and other digital logic circuits, including memory circuits. Other operative circuits 49 in the IPG may be powered directly and only by Vbr or Vbp, as shown in FIG. 3B, such as a resonant tank circuit including telemetry coil (antenna) 36, which tank is coupled to modulation 41 and demodulation 43 circuitry; and a DC-DC converter that generates a power supply V+ for the current generation circuitry (DAC) that produces the stimulation currents at the electrodes 16, as shown in FIG. 3B. FIG. 3B is largely taken from U.S. patent application Ser. No. 13/966,510, filed Aug. 14, 2013, and is incorporated herein by reference. However, operative circuits 47 and 49 can also both be powered by power supply node Vdd.
Control circuitry 38 can comprise a microcontroller integrated circuit, such as MSP430, manufactured by Texas Instruments, which is described in data sheets at that company's website, or as described in U.S. Patent Application Publication 2012/0095529, the latter of which is incorporated herein by reference. Control circuitry 38 may also comprise a microprocessor integrated circuit, a collection of integrated circuits, a collection of non-integrated circuits, or a collection of both integrated and non-integrated circuits—essentially any hardware capable of operating the IPG in the manners disclosed herein.
Various circuits 45 may intervene between Vbr or Vbp provided by batteries 14r or 14p and power supply node Vdd, such as one or more switches used to disconnect the battery in case of a undervoltage or overcurrent condition. See U.S. Patent Application Publication 2013/0023943, which is incorporated herein by reference. Circuits 45 may also include regulators, boost (buck) or step-up (step down) converters, or other conditioning circuits to provide to power supply node Vdd a stable voltage of appropriate magnitude for IPG 10 power supply use.
FIG. 2D shows the external controller 50, such as a hand-held portable patient controller or a clinician's programmer, for communicating with either of IPG 10r or IPG 10p. The external controller 50 typically comprises a graphical user interface similar to that used for a portable computer, cell phone, or other hand held electronic device, including touchable buttons 56 and a display 57, which may also be touch sensitive to allow for patient input. The external controller 50 is used to set or adjust the therapy settings the IPG 10 will provide to the patient, such as which electrodes 16 are active, whether such electrodes sink and source current, and the duration, frequency, and amplitude of pulses formed at the electrodes. The external controller 50 can also act as a receiver of data from the IPG 10, such as various data reporting on the IPG's status, the level of the IPG 10's battery 14r or 14p, and other parameters measured or logged at the IPG 10.
Such communications can occur transcutaneously and bi-directionally via link 75 between a telemetry coil 54 in the external controller 50 and the telemetry coil 36 in the IPG 10, either of which can act as the transmitter or the receiver. Referring to FIG. 3A, when a series of digital data bits is to be sent from the external controller 50 to the IPG 10, control circuitry 60 in the external controller 50 (e.g., a microcontroller) provides these bits in sequence to a modulator 61. Modulator 61 energizes coil 54 with an alternating current (AC) whose frequency is modulated in accordance with the state of the data bit currently being transferred—what is known as a Frequency Shift Keying (FSK) protocol. For example, the coil 54 may nominally be tuned to resonate at 125 kHz in accordance with the inductance of the coil 54 and a tuning capacitor (not shown), with data states ‘0’ and ‘1’ altering this center frequency to f0=121 kHz and f1=129 kHz respectively. The frequency-modulated current through the coil 54 in turn generates a frequency-modulated magnetic field comprising link 75, which in turn induces a frequency-modulated current in the IPG's telemetry coil 36. This received signal is demodulated 43 back into the series of digital data bits, and sent to control circuitry 38 (e.g., a microcontroller) in the IPG 10 for interpretation. Data telemetry in the opposite direction from IPG 10 to external controller 50 via link 75 occurs similarly via modulator 41 and demodulator 62.
Other means for communicating between an external controller and an IPG are known as well, including RF communications such as Bluetooth, Bluetooth Low Energy, Wifi, NFC, Zigbee, etc., that are enabled by patch, wire, or slot antennas. In this instance, link 75 would comprise a longer-range electromagnetic field, rather than a near-field magnetic field enabled by coils 54 and 36. An external controller may comprise a dedicated IPG communication device, or a multi-functional mobile device such as a cell phone, a tablet computer, or another hand-holdable portable control device, as disclosed in U.S. Patent Application Ser. No. 61/874,863, filed Sep. 6, 2013. Optical means of communication may also be used between the external controller and the IPG. See the above-incorporated '877 Application.
Whether an IPG 10r with a rechargeable battery 14r or an IPG 10p with a primary battery 14p is warranted for a given patient depends on weighing several pros and cons associated with each. An IPG with a rechargeable battery can be charged when needed without the need of explantation, but can be more costly, as a charging coil in the IPG and an external charger are required. The need to recharge the rechargeable battery can also be a hassle for a patient. If a patient is missing his external charger, and referring to FIG. 4A, there is a risk that the rechargeable battery may deplete to a voltage (i.e., Vbr=Vbr_min; e.g., 2.0V) insufficient to power the IPG, thus depriving the patient of stimulation therapy. If the voltage of the rechargeable battery becomes lower still and is deeply depleted (i.e., Vbr=Vbr_dd), the patient may be unable to recharge the rechargeable battery with his external charger, and may need to visit a clinician to recover the IPG to a working state. Rechargeable batteries may also suffer from reliability concerns, as they can wear out and work less efficiently as they are cycled over their lifetimes, which can increase the likelihood that a patient will be deprived of therapy. If the rechargeable battery is significantly worn and can no longer hold an adequate charge, there is a possibility that explantation and re-implantation of a fresh IPG will be required.
An IPG with a primary battery does not suffer from these same concerns; for example, there is no additional cost or hassle associated with charging. However, a primary battery IPG will eventually require explantation and re-implantation of a fresh IPG as the primary battery depletes. A curve showing primary battery depletion as a function of time is shown in FIG. 4B, and two significant points are noted. First in time is that corresponding to the issuance of an Elective Replacement Indicator (ERI). ERI issues when the primary battery has sufficiently depleted (i.e., to Vbp_ERI), and will soon reach its End Of Life (EOL). As the primary battery continues to deplete, it will eventually reach EOL, which like Vbr_min described earlier comprises a battery voltage Vbp_EOL insufficient to power the IPG, and at which time therapy will cease.
ERI, when issued, is typically stored at the IPG, and can cause a speaker in the IPG to “beep” to alert the patient that this threshold has been crossed. ERI can also be queried upon a visit to the patient's clinician's office using special wireless monitoring tools, or via telephonic monitoring. ERI is a significant event in the life of a primary battery IPG, as it indicates that the IPG is nearing its EOL and must soon be explanted and replaced. Manufacturers of primary battery IPGs typically design ERI to issue a predetermined time before EOL is reached, such as 2-6 months, to allow a patient sufficient time to schedule necessary replacement surgery. However, the time period between ERI and EOL is not always reliable, and a patient may not be able to schedule surgery quickly enough to have his primary battery IPG replaced before its EOL is reached. Again, this raises the concern that a patient with a primary battery IPG will be deprived therapy.
The inventors are concerned about the possibility that either the primary battery IPG or the rechargeable battery IPG can leave a patient without needed therapy when its battery is sufficiently depleted, and provide solutions to mitigate these concerns.